niUir.UlN(. SYSThMS 



on the stigma. This elcvicc is therefore ahnost as eftectivc as cleisto- 

 gamy in securing inbreeding. Again, in the tomato, poUination 

 follows the opening of the flower, but the stigma is so enclosed in 

 the cone of stamens that it always receives pollen from the same 

 flower. That is, except in stocks where the enclosure is incomplete 

 (or in countries, like its native Peru, where certain insects specialize 

 in cross-pollinating this flower). Some cross-pollination then occurs. 

 British glasshouse commercial varieties are mostly ot a rigorously 

 inbreeding type; American field-grown varieties of the occasionally 

 crossing type. 



Slight genetic variations in the growth of the style (m tomatoes) 

 or time of pollen-shedding (in the cereals) alter the regularity of 

 self-fertiHzation and thus must control the amount of inbreeding. 

 A more extreme effect has been described by Rick in the tomato. 

 He discovered a recessive gene which partiaDy removes the hairs 

 from the plant and, since the cone of anthers is held together by 

 hairs, thereby deprives the flower of its means of regular self- 

 fertilization, and so of some of its fcrtiHty. The absolute rate of 

 outcrossing is unaffected so that, of the reduced amount of seed 

 produced, a greater proportion is crossed; in fact nearly 50 per cent 

 instead of the usual i or 2 per cent. 



Regular cross-breeding, or outbreeding as it is perhaps better 

 called, is more difficult to secure than inbreeding, and is in fact 

 secured by more elaborate devices which act at every stage of the 

 reproductive cycle. The most obvious of these is the formation of 

 unisexual flowers, with the sexes borne on different plants, known 

 as dioecy, or on the same plant, known as monoecy. Dioecy ensures 

 outbreeding. Monoecy only favours it; but its effect may be rein- 

 forced by a timing difference in the production of male and female 

 flowers, as in maize. This timing difference is also common in 

 hermaphrodite flowers. The pollen is shed before [protandry] or after 

 [protogyny) the stigma of the same flower is receptive. Self-fertiliza- 

 tion is thus prevented in the place where, with insect pollination, 

 it is most likely to occur— in the same flower. This timing difference 

 does not, of course, prevent pollination between different flowers on 

 the same plant. In genetic effect this is still, as Darwin showed, 

 self-fertilization, am! the outbreeding mechanism is therefore not 

 fully efficient. 



242 



